1. Field of the Invention
The present invention relates to oxide superconductive wires and a process for manufacturing the same. In particular, the present invention relates to novel oxide superconductive wires and tapes which have improved mechanical strength and excellent superconductive properties, and also to a process for manufacturing such superconductive wires and tapes.
2. Description of the Related Art
Hitherto, various studies have been performed on oxide superconductors containing yttrium (Y), bismuth (Bi) and the like in order to enhance superconductive properties, in particular, characteristics of the JC (the critical current density), and various processes for the manufacture have been proposed.
For example, a composite processing in which starting materials, oxide powders, are filled in a sheath made of pure silver, and subjected to a plasticity processing and a sintering treatment; and a paste method in which oxides in a paste form are applied onto a pure silver base, dried and subjected to a thermal processing are known.
Among such processes, a so-called composite wire processing is regarded as a leading process in the manufacture of superconductive wires containing oxides of Bi. According to this process, the starting Bi-based oxides which had been pre-sintered in air are filled in a pure silver sheath, and then subjected to a plasticity processing to obtain thin wires having various sectional forms. The thin wires thus obtained are rolled into tapes and sintered at a high temperature to produce superconductive wires.
These conventional methods are advantageous in improving the characteristics of the critical current density due to the use of pure silver as a base material such as sheath and the like. However, pure silver materials generally have a low mechanical strength. This causes drawbacks in that when pure silver materials are processed together with starting powder oxides and the like, irregular deformation in the longitudinal direction, which is called sausaging and uneven wall thickness in the cross-section of the wire takes place, which results in clustering of the oxides inside the sheath and breaking of wire.
There is another problem in that the surface matching between the pure silver base material and the bordering oxide layer is not satisfactory.
These problems invite deteriorating superconductive properties, poor characteristics of the critical current density and unreliability.
The above-mentioned composite wire processing which is expected as a promising process for manufacturing oxide-type superconductive wires is not an exception, and involves a drawback in that the silver sheath is softened by the thermal treatment of 800 to 900.degree. C. due to the low melting point of silver. The softened silver sheath is easily deformed plastically under a coiling work of superconductive magnets at room temperature, which raises a problem of unrestorable deterioration of superconductive characteristics.
In order to solve the problems in the mechanical strength, reinforcing or supporting materials have conventionally been employed. In this case, however, critical current density per unit wire cross-section is reduced, which is unfavorable because it hinders a manufacturing of compact and light superconductor apparatus.
Under the above circumstances, the present inventors have conducted earnest studies in an attempt to overcome drawbacks involved in conventional processes for producing superconductive wires where pure silver is used as a base material of a sheath and the like, and to provide a process for producing oxide superconductive wires which have excellent mechanical strength, superconductivity and improved plastic workability, leading to completion of the invention.